It is well known in the field of sound barrier walls that the effectiveness of a single wall is proportional to its surface density (weight per unit area) and that the effectiveness of a single wall increases in proportion to the frequency of the sound. Thus, in order to exclude low frequency sound very massive walls are required.
Since, however, some sound barrier applications cannot tolerate great weight, such as the sidewalls of passenger transport aircraft which must exclude intense exterior sound but at the same time must be as light as possible, the need for lighter weight walls led to the development of compound wall structures. It was found that in the range of medium to high frequencies a compound wall, comprising a relatively light outer wall, such as a fuselage skin, and a light inner wall, such as a trim panel, spaced apart with an intervening airspace, could provide considerably greater sound reduction for a given total weight. Further improvement in reduction could be obtained if sound absorptive material such as a fiberglass blanket was placed in the airspace between the inner and outer walls. These blankets also served to provide essential thermal insulation and are known as thermoacoustical blankets. Small additional benefits were sometimes obtainable by subdivision of the thermoacoustical blanket into several layers by means of thin flexible sheets (septa). These compound walls were intensively developed and attained a high degree of refinement, and are used universally on transport aircraft powered by jet or turbofan engines.
The compound walls which serve to exclude the middle and high frequency sound from transport aircraft cabins are, however, actually inferior to a single wall of the same weight at lower frequencies. Moreover, single walls designed to exclude, for example, the intense blade passage frequencies produced by high performance propeller driven aircraft would be prohibitively heavy and the required weight would negate much if not all of the potential fuel savings attainable with this type of propulsion.
Accordingly, it is a general object of the present invention to provide an improved sound barrier.
It is another object of the present invention to provide a lightweight sound barrier with increased transmission loss effectivity.
It is a further object of the present invention to provide a sound barrier which produces augmented sound reduction at selected frequencies.
It is still another object of the present invention to provide a sound barrier which incorporates resonant acoustical elements.
It is a further object of the present invention to provide a sound barrier suitable for the construction of fuselage walls for propeller driven aircraft.